In general, undesired or non-ideal characteristics, such as DC offset and in-phase/quadrature-phase (IQ) imbalance, degrade performance of mobile transceivers. The DC offset is the effect of self mixing by a mixer, and occurs when a signal of a local oscillator (LO) returns after leaking toward an antenna or when a radio frequency (RF) modulation signal input through the antenna is leaked to the local oscillator. Another way to create DC offset is through an inherent offset in the amplifiers due to imbalances. If the DC offset is amplified by amplifiers in the signal path, then this way may saturate a baseband circuit.
The IQ imbalance is caused when the phase difference between the in-phase (I) channel signal and the quadrature-phase (Q) channel signal generated in an oscillator of a wireless transmitter is not 90 degrees. The IQ imbalance can be reduced by designing mixers of the I channel demodulator and the Q channel demodulator to be precisely 90 degrees in phase delay (i.e., orthogonal) to each other. However, designing the mixers so that there is precisely a 90 degrees phase difference to each other is not practical over process and temperature variations. This is because in the layout, the I and Q paths to the mixers traverse different lengths despite the best effort of keeping everything symmetrical. This is especially true for multi-band systems. An IQ imbalance increases the Bit Error Rate (BER), thereby degrading the performance of the wireless transceiver.
One approach for compensating DC offset and IQ imbalance between orthogonal signals within in a mobile wireless communications device is disclosed in U.S. Pat. No. 7,782,928. The communications device includes a transmitter that functions as a signal generator, and a receiver that functions as a response characteristic detector. A baseband processor applies predefined test signals to the transmitter, receives the test signals returning from the receiver, and compensates the imbalance and DC offset for the transmitter side and the receiver side by using the test signals.
Another approach for compensating DC offset and IQ imbalance between orthogonal signals within in a mobile wireless communications device is disclosed in U.S. Published Patent Application No. 2009/0262861. A baseband processor generates an I baseband signal and a Q baseband signal. A direct up-converter is coupled to the baseband processor, and combines the I and Q baseband signals with an RF carrier signal to generate an RF output signal. The antenna is coupled to the direct up-converter, and transmits the RF output signal. An impairment detection and compensation feedback circuit is coupled to the RF output signal, and the I and Q baseband signals. The impairment detection and compensation feedback circuit down-converts the RF output signal to generate an intermediate frequency (IF) signal, measures as least one signal impairment in the IF signal, and pre-distorts the I and Q baseband signals to compensate for the measured signal impairment.
Yet another approach is disclosed in the article titled “Calibration of Direct-Conversion Transceivers” by Debaille, IEEE Journal of Selected Topics in Signal Processing, Volume 3, No. 3, June 2009. To enable separation of the impairments caused by the transmitter and the receiver, their local oscillators are operated at slightly different frequencies. Compensation is then based on using a dual-tone calibration signal, such as a standard compliant OFDM modulated preamble, wherein a multi-tone symbol travels through a transceiver configured in loop-back. The compensation is performed by pre- and post-compensating the baseband signal of the transmitter and receiver respectively.
Even in view of the above approaches for compensating DC offset and IQ imbalance between orthogonal signals within in a mobile wireless communications device, there is still a need to improve such compensation. A drawback of the above approaches is that data cannot be transmitted by the transmitter when being trained or compensated.